1. Technical Field
The present invention, generally, relates to endoprosthesis, and more particularly, it relates to a new stent, a stent in combination with an angioplasty balloon, tooling for forming a stent, and a method of fabricating a stent.
2. Related Art
Stents are devices used to support the walls of weak arteries and are particularly useful in the medical field of angioplasty involving the reconstruction of vessels that carry blood in both humans and animals. The stent is used to maintain such blood vessels, structurally, in a clear and open condition. Most arterial stents are formed from thin wire, e.g., 0.005 inch diameter wire. Stents are often made from inert metals such as stainless steel or tantalum. However, plastic stents are available and provide more flexibility. Unfortunately, plastic stents also lack sufficient radial stiffness for artery wall support.
For a stent to achieve maximum usefulness, it must be flexible in a bending mode during insertion, and it must exhibit stiffness in both torsional and cylindrical modes in order to provide support. The stents today are formed into the required configuration to permit a high level of plastic deformation to be achieved during their use. During use, an angioplasty balloon may be inserted into a stent which is then plastically compressed around the balloon. This assembly is inserted into a patient's blood vessel, usually an artery, and moved into position. The balloon is inflated to enlarge the stent to a desired diameter, after which the balloon is removed.
The stent, within an artery, or within any other type of vessel, is exposed to repetitive flexing as a part of a circulatory system, both from the systolic and the diastolic variations in blood pressure and from variations in movement of a body.
One method of stent fabrication today continuously feeds a wire from a spool to be formed into a generally sinusoidal configuration. Then, the wire in this sinusoidal configuration is wound around a mandrel in order to produce a helical arrangement. Next, the crests and troughs in the helical arrangement are aligned so that they touch at points, and then crests and troughs are welded at the points to provide the required supporting structure. The above described steps, however, in conjunction with the heating and cooling encountered during welding, create an undesirable work-hardening in the wire. This work hardening lowers much of the wire's ability to provide support in use. Additionally, loading and unloading of a metallic stent during use produces further fatigue of the metal, causing premature failure of support.
An additional disadvantage of present fabrication methods is that the deformation of the wire into a desired position is also relatively imprecise such that a uniformly shaped stent is difficult to create. Further, since each joint is welded individually, inconsistency of structure can be created. As a result, the stents so fabricated also lack in consistency of torsional and radial stiffness. The need to deform the wire into a given position and then join the wire also involves very high costs.
Examples of related art, which are hereby incorporated by reference, are:
U.S. Pat. No. 5,370,683 to Fontaine and assigned to Cook, Inc. describes a stent formed of a single filament wrapped around a mandrel with a series of U-shaped bends. PA1 U.S. Pat. No. 5,304,200 to Spaulding and assigned to Cordis Corp. describes a method of making stents involving winding an elongated strand forming a helix like structure with the ends welded to an adjacent section. PA1 U.S. Pat. No. 5,217,483 to Tower and assigned to Numed, Inc. describes a stent arranged to have U-shaped sections formed in a continuous wire with two ends and with the ends attached together to prevent axial expansion. PA1 U.S. Pat. No. 5,549,663 to Cottone, Jr. and assigned to Cordis Corp. describes a stent formed by wrapping a wire around a mandrel and joining the filaments by welding. PA1 U.S. Pat. No. 5,629,077 to Turulund et al. and assigned to Advanced Cardiovascular Systems, Inc. describes a stent made completely of biodegradable material. PA1 U.S. Pat. No. 5,630,829 to Lauterjung and assigned to InterVascular, Inc. describes a stent in which adjacent stents may be connected to one another by welding at least one opposed pair of cusps together.
Accordingly, there is a need for a stent which consistently exhibits flexibility during insertion, and stiffness with a high level of plastic deformation in both torsional and radial modes to provide support during use. Further, there is a need for a stent fabrication process and tooling which creates the above described stent more easily than conventional methods and without the problem of work hardening and high cost.